When steam flooding a subsurface petroleum-containing formation, steam is usually generated at the earth's surface and injected into the subsurface formation from a cased well. The steam is usually transported to the producing formation through a conduit run inside the casing within the well. In a successful steam flood operation, the injection of steam into the target formation is maximized, while the loss of heat from the steam as it is transported from the surface to the target formation through the well conduit is minimized. Thermal insulation between the steam carrying conduit and the formation has been proposed as a means for reducing heat loss from the steam conduit to the formation. The construction of insulated conduit sections and the joining of sections of such insulated conduit into a concentric insulated tubing string is the subject matter of this invention.